The invention is directed to a soot blower for cleaning the interior surfaces of a boiler by discharging a suitable cleaning fluid such as air, water or steam from a nozzle or nozzles arranged at one end of a rotatable and retractable lance tube and toward said boiler surfaces. More particularly, the invention relates to a new and improved transition weld between a "heavier" wall section and a "thinner" wall section of the lance tube.
Typically, in a soot blower of the long retracting type, a lance tube is moved through a long, e.g., 55 foot path of travel horizontally forward into the heat exchange zone of a large public utility boiler. During the traveling motion of the lance tube, the tube is rotated about its longitudinal axis and a cleaning fluid is discharged through a nozzle or nozzles mounted at the forwardmost end of the tube so that the fluid may be directed against various internal surfaces of the boiler to remove undesirable soot accumulations. In a conventional soot blower, the lance tube is mounted on a traveling carriage which securely mounts one end of the lance tube and which is operable to impart the translatory and rotational movement to the lance tube and guide the long lance tube as it enters and leaves the boiler.
As illustrated and described in more detail below, the physical arrangement between the lance tube, traveling carriage and other components of the soot blower requires the lance tube to be of a uniform outer diameter for its entire length of up to 55 feet. The long cantilever configuration of the lance tube within the soot blower necessitates proper selection of diameter and wall thickness to minimize both bending stress caused by the cantilever configuration and torsional stress caused by the torque generated about the one supported end of the lance tube. Typically, the lance tube comprises two or more hollow, tubular sections of varying wall thicknesses. The sections are arranged relative to one another such that the section having the thinnest wall section is located at the unsupported end of the lance tube to reduce weight and minimize stress on the lance tube and the section having the thickest wall section is located at the end of the lance tube supported by the traveling carriage for maximum strength.
It is a primary objective of the present invention to provide a new and improved transition weld of a thicker wall section to a thinner wall section while maintaining a uniform outer diameter for the entire length of the lance tube as well as a smooth surface throughout the interior of the lance tube. In accordance with conventional practice, the smooth internal surface is accomplished by machining the heavy wall inner diameter to match the thin wall inner diameter. However, a problem associated with the conventional transition is that the internal grinding required after grinding to provide a smooth internal surface is difficult to control because it is performed at locations ten to thirty feet within a 4 to 5 inch diameter lance tube. Consequently, the difficult to control grinding very often reduces the cross section of the thin wall section unacceptably below the original inner diameter of the thin wall tube.
Pursuant to the invention, the end of the thin wall section to be welded to the thick wall section is formed by swaging or other suitable means. In this manner, the inner diameter of the thin wall section is arranged to match the inner diameter of the thick wall section at the transition area and weld filler is utilized to achieve a uniform outer diameter between the thick wall section and the formed end of the thin wall section. It has been discovered through empirical testing that the stress developed about the transition weld in accordance with the invention is considerably less than the stress about a conventional transition weld joint. Accordingly, the invention provides a weld transition that is inherently more suitable to secure two tubular sections under both bending and torsional stress to one another than the heretofore utilized conventional weld.
In addition, it should be understood that the lance tube operates in a high temperature environment within the boiler. The cleaning fluid passing through the lance tube cools the lance tube, particularly the section closest to the traveling carriage, inasmuch as the cleaning fluid is coolest and more able to absorb heat when it first enters the lance tube. Accordingly, it is common to use a material for the thick wall section mounted to the traveling carriage different than the material used for the thin wall section at the free, nozzle end of the tube. The material for the thin wall section generally comprises an alloy having a higher tensile strength at higher temperatures than the alloy utilized for the thick wall section. An important additional advantage of the invention is that the thick wall section is not machined down to match the inner diameter of the next thinner wall section. Therefore, the thick wall section is generally uniformly of the same wall thickness throughout its entire length and not susceptible to a critical weak point due to a declining tensile strength at high temperatures near the transition as might occur with the conventional machined, reduced thickness end of the thick wall section, as will appear.
Thus, the present invention provides a highly advantageous improvement in the construction of multi-section lance tubes for soot blowers of the long retracting type. The transition weld of the invention is easier to achieve, inherently better able to endure bending and torsional stress and subject to improved tensile strength within the high temperature environment of its operation. Accordingly, a lance tube built in accordance with the teachings of the invention will be much more resistant to a primary problem of soot blower operation, i.e., the bending or breaking off of the lance tube within the boiler, usually at the weld joint.
For a better understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawings.